Mapping the Early Universe in 3 Dimensions

The invention of the CAT scan led to a revolution in medical diagnosis. Where X-rays give only a flat two-dimensional view of the human body, a CAT scan provides a more revealing three-dimensional view. To do this, CAT scans take many virtual “slices” electronically and assemble them into a 3D picture.

Now a new technique that resembles CAT scans, known as tomography, is poised to revolutionize the study of the young universe and the end of the cosmic “dark ages.” Reporting in the Nov. 11, 2004, issue of Nature, astrophysicists J. Stuart B. Wyithe (University of Melbourne) and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics) have calculated the size of cosmic structures that will be measured when astronomers effectively take CAT scan-like images of the early universe. Those measurements will show how the universe evolved over its first billion years of existence.

“Until now, we’ve been limited to a single snapshot of the universe’s childhood-the cosmic microwave background,” says Loeb. “This new technique will let us view an entire album full of the universe’s baby photos. We can watch the universe grow up and mature.”

Slicing Space
The heart of the tomography technique described by Wyithe and Loeb is the study of 21-centimeter-wavelength radiation from neutral hydrogen atoms. In our own galaxy, this radiation has helped astronomers to map the Milky Way’s spherical halo. To map the distant young universe, astronomers must detect 21-cm radiation that has been redshifted: stretched to longer wavelengths (and lower frequencies) by the expansion of space itself.

Redshift is directly correlated to distance. The farther a cloud of hydrogen is from the Earth, the more its radiation is redshifted. Therefore, by looking at a specific frequency, astronomers can photograph a “slice” of the universe at a specific distance. By stepping through many frequencies, they can photograph many slices and build up a three-dimensional picture of the universe.

“Tomography is a complicated process, which is one reason why it hasn’t been done before at very high redshifts,” says Wyithe. “But it’s also very promising because it’s one of the few techniques that will let us study the first billion years of the universe’s history.”

A Soap Bubble Universe
The first billion years are critical because that is when the first stars began to shine and the first galaxies began to form in compact clusters. Those stars burned hotly, emitting huge amounts of ultraviolet light that ionized nearby hydrogen atoms, splitting electrons from protons and clearing away the fog of neutral gas that filled the early universe.

Young galaxy clusters soon were surrounded by bubbles of ionized gas much like soap bubbles floating in a tub of water. As more ultraviolet light flooded space, the bubbles grew larger and gradually merged together. Eventually, about a billion years after the Big Bang, the entire visible universe was ionized.

To study the early universe when the bubbles were small and the gas mostly neutral, astronomers must take slices through space as if slicing a block of swiss cheese. Loeb says that just as with cheese, “if our slices of the universe are too narrow, we’ll keep hitting the same bubbles. The view will never change.”

To get truly useful measurements, astronomers must take larger slices that hit different bubbles. Each slice must be wider than the width of a typical bubble. Wyithe and Loeb calculate that the largest individual bubbles reached sizes of about 30 million light-years across in the early universe (equivalent to more than 200 million light-years in the expanded universe of today). Those crucial predictions will guide the design of radio instruments to conduct tomographical studies.

Astronomers soon will test Wyithe and Loeb’s predictions using an array of antennas tuned to operate at the 100-200 megahertz frequencies of redshifted 21-cm hydrogen. Mapping the sky at these frequencies is extremely difficult because of manmade interference (TV and FM radio) and the effects of the earth’s ionosphere on low-frequency radio waves. However, new low-cost electronics and computer technologies will make extensive mapping possible before the end of the decade.

“Stuart and Avi’s calculations are beautiful because once we have built our arrays, the predictions will be straightforward to test as we take our first glimpses of the early universe,” says Smithsonian radio astronomer Lincoln Greenhill (CfA).

Greenhill is working to create those first glimpses through a proposal to equip the National Science Foundation’s Very Large Array with the necessary receivers and electronics, funded by the Smithsonian. “With luck, we will create the first images of the shells of hot material around several of the youngest quasars in the universe,” says Greenhill.

Wyithe and Loeb’s results also will help guide the design and development of next-generation radio observatories being built from the ground up, such as the European LOFAR project and an array proposed by a US-Australian collaboration for construction in the radio-quiet outback of Western Australia.